Polyarylene sulfide (hereafter, may be abbreviated as PAS) represented by polyphenylene sulfide (hereafter, may be abbreviated as PPS) is a resin which has preferable properties such as excellent heat resistance, barrier properties, chemical resistance, electric insulation, hot wet resistance or flame resistance, as an engineering plastic. It is moldable by injection molding or extrusion molding into various molded parts such as a film, a sheet or a fiber, and widely used in the fields such as of various electric electronic parts, mechanical parts and automobile parts in which heat resistance and chemical resistance are required.
As a production method of PAS, a method of reacting alkali metal sulfides such as sodium sulfide and polyhalogenated aromatic compounds such as p-dichlorobenzene in organic amide solvents such as N-methyl-2-pyrrolidone is known, and this method is widely employed as an industrial production method of PAS. However, in that production method, it is necessary to carry out the reaction at a high temperature, a high pressure and under a strong alkaline condition, and further, expensive polar solvents of high boiling point such as N-methyl pyrrolidone are necessary, and it has a problem that a great process cost is needed since it is energy exhaustive type in which a great cost is needed for solvent recycling.
Furthermore, since the polymerization reaction is of a polycondensation mechanism, side products such as sodium chloride are produced. Accordingly, a removing step of the byproduced salt is necessary after the polymerization reaction. However, it is difficult to completely remove the byproduced salt by ordinary treatment, and in commercially available and widely used PPS products, about 1,000 to 3,000 ppm in alkali metal content is contained. When an alkali metal salt is left in the produced polymers, such problems as damaging physical characteristics including electric characteristics arises. Accordingly, when a molded article in which such a polyarylene sulfide is used as the raw material is used in the field of electric or electronic parts, deterioration of electric characteristics by the alkali metal in the polyarylene sulfide becomes a big obstacle.
Furthermore, polyarylene sulfide obtained by that method contains low molecular weight components, wherein the polydispersity expressed by the ratio of weight average molecular weight and number average molecular weight is very large and it is a polymer of a broad molecular weight distribution. For that reason, when it is used for a mold processing application, sufficient mechanical characteristics are not exhibited, and problems arose that an amount of gas component when heated was large, soluble amount when contacted with a solvent was large, etc. To solve these problems, for example, a step of forming a cross-linking structure to thereby convert it to a high molecular weight by subjecting to a vapor phase oxidation treatment under oxidizing atmosphere such as in the air, becomes necessary, but the process became further complicated and productivity also decreased (e.g., refer to JP-S45-3368 B (pages 7 to 10)).
As methods of improving one of the above-mentioned problems of PAS, that is, as a method to improve that the molecular weight distribution is broad since PAS contains low molecular weight components, a method of purification by phase separating a mixture of PAS containing impurities to a molten polymer phase containing PAS and a solvent phase mainly comprising solvent at a temperature higher than the lowest temperature at which PAS forms a molten phase, to thereby heat extract the impurities, or a method of precipitating and collecting granular polymer after cooling, is known. In that method, it is expected that the metal content of PAS is decreased and the molecular weight distribution is narrowed since the impurities are extracted by the heat extraction effect, but the effect was insufficient, and, the process was complicated since it was a method in which an expensive solvent were used (e.g., JP-H1-25493 B (page 23) and JP-H4-55445 B (pages 3 to 4)).
As another method for solving the above-mentioned problem, that is, the problem that PAS contains low molecular weight components and has a broad molecular weight distribution, a PAS of which ratio of weight average molecular weight Mw and number average molecular weight Mn, Mw/Mn, is in the range of 2 to 5, which is produced by a method characterized in that a sulfur source and dihaloaromatic compound were reacted in an organic polar solvent under a condition of a temperature 220 to 280° C. for 0.1 to 2 hours and the PAS obtained was washed with an organic polar solvent under a condition of a temperature 100 to 220° C., is disclosed. In that production of PAS, since PAS of narrow molecular weight distribution is obtained by removing low molecular weight components by washing with an organic solvent at a high temperature, the yield of PAS was low, and, even in the PAS of the lowest polydispersity substantially obtained, Mw/Mn=2.9, and the effect was insufficient. Furthermore, in that method, since expensive lithium compound is used in the polymerization of PAS, there remained many problems to be solved, such as that it is not economical or the lithium remains in PAS considerably (e.g., refer to JP-H2-182727 A (pages 9 to 13)).
As a method of improving the insufficient molecular weight distribution of the above-mentioned PAS, a PAS obtained by a production method characterized in that, an alkali metal sulfide and/or an alkali earth metal sulfide and a polyhalogenated aromatic compound are polymerized in an aprotic organic solvent, and to the obtained polymerization solution containing PAS polymerization reaction product, 5 to 50 wt % water with respect to the whole solution and an inorganic and/or organic acid to acidify the above-mentioned polymerization solution are added, and at a temperature higher than the lowest temperature at which the PAS polymerization reaction product forms a molten phase, the polymerization solution is phase separated to a solvent phase and a molten polymer phase to thereby the molten polymer phase is collected, is disclosed. By this method, it is expected that a PAS of a narrow molecular weight distribution having a polydispersity, Mw/Mn, around 1.9 is obtained. However, in that method, since a method of separating low molecular weight components from PAS is employed by subjecting PAS having a broad molecular weight distribution to an extraction treatment in a molten state at a high temperature and a high pressure, the process is complicated and the yield of PAS obtained is low as 80% or less. Further, in that polymerization of PAS, expensive lithium compound is used, and there remains a problem that residual lithium in the polymer may be present (e.g., JP-H9-286860 A (pages 5 to 6)).
Furthermore, as a production method of PAS having a narrow molecular weight distribution, a method of heating ring-opening polymerization of a cyclic arylene sulfide oligomer in the presence of an ionic ring-opening polymerization catalyst, is known. In that method, which is different from the above-mentioned JP-H1-25493 B (page 23) and JP-H4-55445 B (pages 3 to 4), it is expected that a PAS having a narrow molecular weight distribution is obtained without the complicated organic solvent cleaning operation. However, in that method, since an alkali metal salt of sulfur such as sodium salt of thiophenol is used as the ring-opening polymerization catalyst in the synthesis of PAS, there was a problem that much amount of alkali metal remains in the PAS. In the case where the amount of the residual alkali metal is decreased by decreasing the amount used of the ring-opening polymerization catalyst, there was a problem that molecular weight of PAS obtained becomes insufficient (e.g., JP Patent No. 3216228, specification (pages 7 to 10), and U.S. Pat. No. 3,141,459, specification (pages 5 to 6)).
As a method of solving the problem of PAS obtained by the above-mentioned method, i.e., a method of decreasing the amount of residual alkali metal in PAS, a production method of PAS in which a cyclic aromatic thioether oligomer is ring-opening polymerized by heating under presence of a polymerization initiator which generates a sulfur radical, is known. In that method, since a nonionic compound is used as a polymerization initiator, it is expected that an alkali metal content of the obtained PAS is decreased. However, glass transition temperature of the polyphenylene sulfide obtained by that method is low at 85° C. This is because the molecular weight is low and the polyphenylene sulfide contains low molecular weight components and the molecular weight distribution is broad, and it was insufficient in view of molecular weight and narrow molecular weight distribution. Furthermore, although there is no disclosure about a weight loss ratio when heated of the polyphenylene sulfide obtained by the method, since the polymerization initiator used in the method has a lower molecular weight compared to that of the polyphenylene sulfide and its heat stability is also inferior, it was feared that much amount of gas is generated when the polyphenylene sulfide obtained by that method is heated, and its molding processability is poor (e.g., U.S. Pat. No. 5,869,599, specification (pages 27 to 28)).
Furthermore, in the ring-opening polymerization in the production method of PAS disclosed in JP Patent No. 3216228, specification (pages 7 to 10), U.S. Pat. No. 3,141,459, specification (pages 5 to 6), and U.S. Pat. No. 5,869,599, specification (pages 27 to 28), it is described that it is preferable to use a high purity cyclic polyarylene sulfide oligomer which substantially does not contain a linear polyarylene sulfide as its monomer source, and only an extremely small amount of linear polyarylene sulfide is admitted. In general, since cyclic oligomer is obtained as a mixture with much amount of linear oligomer, a purification operation of high level is necessary to obtain a high purity cyclic oligomer, and this becomes a cost factor relative to the amount of PAS obtained. Thus, a method which allows the use of a mixture of cyclic oligomer and linear oligomer as a monomer source in the ring-opening polymerization has been desired.
On the other hand, a polymerization method of PPS in which a mixture of cyclic PPS and linear PPS is heated as monomer source is also known (POLYMER, Vol. 37, No. 14, 1996 (pages 3111 to 3116)). This method is an easy polymerization method of PPS, but the degree of polymerization of the obtained PPS was low and it is a PPS which is improper for practical use. That publication discloses that the degree of polymerization could be increased by raising the heating temperature, but yet it did not arrive at a molecular weight suitable for practical use, and in this case, formation of a cross-linked structure could not be avoided and it was pointed out that only a PPS which is poor in thermal characteristics could be obtained. Thus, a polymerization method of a PPS of high quality suitable for practical use has been desired.
On the other hand, as methods of decreasing weight loss ratio when heated of PAS, conventionally many proposals were made regarding heat treating method of PAS, for example, a method of heat treating polyphenylene sulfide under oxygen atmosphere at a temperature lower than its melting point, or a method of heat treating polyphenylene sulfide in an inert gas atmosphere at a temperature lower than its melting point, etc. (e.g., U.S. Pat. No. 3,793,256, specification (page 2), and JP-H3-41152 A (claims)). Although in the polyarylene sulfide obtained by these methods, in fact, compared to the PAS which is not subjected to a heat treatment, weight loss ratios when heated are apt to decrease, but yet their weight loss ratios when heated are not at a level which can be satisfied. Furthermore, the polyphenylene sulfide obtained by that method contains low molecular weight components, and its polydispersity which is expressed by the ratio of weight average molecular weight and number average molecular weight is very large. Thus, the problems that it is a polymer of which the molecular weight distribution was broad, and is a polymer of low purity of which alkali metal content is very high, were not solved.
As another method of heat treating PAS, a PAS resin pellet obtained by melt-extruding while purging a vent port with nitrogen while keeping a vent port under a reduced pressure when PAS resin is melt-extruded by using an extruder equipped with a vent port, is known (e.g., JP 2000-246733 A (page 4)). In that method, since the heat treatment is carried out at melting temperature of PAS or more under a reduced pressure condition, a degassing effect during the heat treatment is excellent and it is expected that the weight loss ratio when heated of the PAS obtained by that method is decreased, but its level was not yet a level which could be satisfied. Furthermore, the PAS pellet according to that method had also a molecular weight distribution characteristics and alkali metal content characteristics similar to those of the above-mentioned polyphenylene sulfide, and it could not be said that the purity was sufficiently high.
It could therefore be advantageous to provide polyarylene sulfides having a narrow molecular weight distribution and of a high molecular weight, a high purity and industrially useful, and production methods of polyarylene sulfide having such advantages.